ISTANBUL — With airstrikes and artillery fire, Turkey on Saturday defied U.S. appeals and opened a long-anticipated offensive on ­Afrin, an enclave in Syria for Kurdish militias backed by the United States.

Turkish officials have framed the offensive as part of a wider battle against Kurdish separatists, known as the Kurdistan Workers' Party, in Turkey's southeast. Turkey also fears any gains in strength by the Syrian Kurds, whose territory runs along some of Turkey's southern border.

But the United States has opted to back the Syrian Kurds as proxy fighters against the Islamic State and as a buffer to keep the militants from trying to reclaim territory.

The military action immediately raised concerns that it could spark conflicts among the assortment of foreign military powers present, in proximity, across northern Syria. They include Turkey, Russia and the United States. All have the Islamic State as a common foe, but, individually, they back different factions among the various armed groups in Syria.

The latest flash point also highlighted the shifting disputes and conflicting agendas that have complicated any efforts toward ending nearly seven years of conflict in Syria. The Turkish military action came amid intensifying violence in the northern Syrian province of Idlib, where Syrian government forces are on the offensive against al-Qaeda-aligned rebels in the east of the province.

Recent statements by U.S. military officials about plans to train border security forces that would protect a Kurdish enclave in Syria also provoked Turkey's ire.

"We are taking these steps to ensure our own national security," President Recep Tayyip Erdogan said in comments carried by the semiofficial Anadolu agency.

Yet Turkish incursions could carry risks. The government of Syrian President Bashar al-Assad had warned that it was prepared to fire on Turkish warplanes in the event of an attack on Afrin.

A Syrian government offensive is causing one of the worst surges in population displacement since Syria's civil war began. More than 212,000 people have fled fighting around Idlib in the past month, many of them sleeping in the open as temperatures plunge and rain drenches makeshift campsites, according to the United Nations.

On Saturday, hours after the announcement of the airstrikes, Turkey said it had struck more than 100 positions belonging to Kurdish fighters. The number of casualties was not immediately clear. The airstrikes followed days of intense Turkish artillery fire on Kurdish positions, according to residents in Afrin.

In a statement, the U.S.-backed Kurdish force, known as the Syrian Democratic Forces, warned that the Turkish offensive "threatens to breathe new life into Daesh," using an Arabic acronym for the Islamic State militant group.

The Trump administration, in urging NATO-ally Turkey not to attack, had made a similar argument, saying it would distract from the ongoing battles against Islamic State militants in their remaining strongholds in Syria. There are roughly 2,000 U.S. troops in northern Syria.

Russia, which backs Assad's government, said it was watching developments "with concern" and called on the warring sides to "exercise mutual restraint." Russia's Defense Ministry said that an unspecified number of Russian troops had been moved out of the Afrin area and redeployed.

"The challenge is that no one knows what they intend to do," said Aaron Stein, a resident senior fellow at the Atlantic Council's Rafik Hariri Center for the Middle East.

"Afrin will be hostile to a Turkish-backed force patrolling from permanent garrisons. The YPG in the area can retreat to the mountains for protection," he said, referring to the Syrian Kurdish militia that controls Afrin.

The offensive probably was prompted in part by Turkish concerns that Russia and the United States planned to broker a reconciliation between Syria's government and the Syrian Kurdish forces. "This is anathema to Turkey for obvious reasons," Stein said. "So they are making a statement."

Loveluck reported from London. Suzan Haidamous in Beirut, Heba Habib in Stockholm and Anton Troianovski in Moscow contributed to this report.